Define lsst_apps, lsst_obs, lsst_distrib in pipelines.lsst.io

Details

Description

We do not currently have a definition of the lsst_apps, lsst_obs, and lsst_distrib meta-packages in our documentation. It seems to me that these meta-packages are for the convenience of organizing the Stack code and might change from release to release, so they should be defined in pipelines.lsst.io. (A possible alternative might be to define them by fiat in LDM-148, but I think that is less desirable.) I propose to do so at the beginning of https://pipelines.lsst.io/install/index.html.

lsst_apps contains the Science Pipelines algorithmic code that we expect to use in production and its dependencies, including any packages needed to test and verify the others, such as a minimal set of obs_* observatory/camera definition packages.

lsst_obs contains all supported obs_* packages.

lsst_distrib contains additional LSST-supported software related to the Alert, Calibration Products, and Data Release Productions, including the following not in lsst_apps or lsst_obs:

Optional algorithmic code and plugins not expected to be used in production or testing

Production control software and configurations for organizing large-scale computations